Methods of fabricating a plotter drum

ABSTRACT

A drum for a photoplotter has a base having a partially cylindrical recess formed therein which recess being defined by in part by a plurality of elevated surfaces providing a support bed on which is mounted a sheet of material providing an exposed surface for supporting a substrate thereon. The drum is formed by a method whereby a tool is provided about which the sheet material is drawn and is moved into engagement with the bed and is caused to maintain this shape by an adhesive interposed therebetween. A method for mechanically compensating for surface irregularities by fixing the scanner relative to the support surface after displacements have been made is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of co-pending application Ser. No. 07/839,398,filed on Feb. 20, 1992.

This application relates to co-pending U.S. application Ser. No.07/838,500, filed on Feb. 19, 1992, in the name of H. Joseph Gerber andentitled A Method and Apparatus for Enhancing the Accuracy of ScannerSystems, which application being commonly assigned with the assignee ofthe present invention. This application has been issued as U.S. Pat. No.5,291,392.

BACKGROUND OF THE INVENTION

This invention relates to internal drum photoplotters and the support onwhich a substrate is held during a plotting operation, and deals moreparticularly with a drum for such plotters and an associated method ofmaking the same wherein the drum can be readily fabricated with asurface having a highly cylindrical shape; the invention further dealswith a method of mechanically compensating for surface irregularitieswhich may otherwise occur by precisely adjusting the orientation of thescanning means relative to the support surface.

Internal drum photoplotters utilize a section of a cylindrical surfaceto support a photosensitive film during a plotting operation. Thissurface must be as perfectly cylindrical as possible to insure thehighest degree of plotting accuracy achievable on the film orphotosensitive substrate supporting it. Any irregularity in this surfacewill cause an undesirable variance to be made in the lines projectedonto the substrate. The need to provide a surface having as fewirregularities as possible is to some extent also recognized in flat bedphotoplotters. But, since a planar support surface is involved in thiscase, providing such a conforming surface is more easily effected thanone having a cylindrical shape. For example, in U.S. Pat. No. 4,963,217,issued to Robert J. Pavone on Oct. 16, 1990, and being commonly assignedwith the assignee of the present invention, it is disclosed to form anaccurate planar surface by using a tool presenting a highly accurateflat surface to be transferred to the support surface of the platenconstituted by sheet material. This is done by applying a layer ofadhesive to one side of the sheet and placing it face down on itsopposite side onto the tool. Subsequently, a honeycomb infrastructure ismoved down into the exposed adhesive, such that when the adhesive dries,the support surface is held in the desired planar condition by theinfrastructure, regardless of whether it entirely seats flushly againstit. By contrast, to fabricate a section of a cylindrical surface in adrum plotter using sheet material, a more complicated process must beemployed because the supporting drum base presents a surface which iscurved requiring that the sheet be maintained in a curved conditionduring the attaching process. Further complicating this process, is thevacuum system provided in the base for dispersing vacuum evenly belowthe support surface so that the film can be drawn against the contour itpresents. Such a system must be one in which the sheet materialconstituting the support surface is held rigidly in a cylindrical shapewhile nevertheless being capable of permitting the free flow of aircommunicating through the surface and thereafter through the base.

Accordingly, it is an object of the present invention to provide a drumfor use in an internal drum raster photoplotter which can be readilyfabricated so as to provide a support surface which is highlycylindrical for holding and supporting a substrate during a plottingoperation.

It is a further object of the present invention to provide a drumsupport of the aforementioned type wherein the drum is made by a processallowing the support surface to be readily assembled with the highestpossible degree of accuracy.

The invention further resides in a method of fixturing the scanner meansof the photoplotter to account for defects or irregularities that mayexist in the surface in order to minimize the need to electronicallycompensate for such variances in the scanning process.

Further objects and details of the invention will become apparent fromthe following description and appended claims.

SUMMARY OF THE INVENTION

The invention resides in a drum support for use in a photoplotter havinga two part construction comprising a sheet of material having onesurface and an opposite other surface, with the one surface adapted forsupporting an article thereon and a body portion having a partiallycylindrical recess formed therein oriented about a given longitudinalaxis, the recess having means for presenting a plurality of elevatedsurfaces together defining the partially cylindrical recess andproviding a bed for supporting the other surface of the sheet ofmaterial. The sheet material and the body portion are attached by meansinterposed therebetween and a means is provided in the sheet materialfor communicating air between the one surface of the sheet material andthe body portion of the drum.

The invention further resides in a method of forming a highlycylindrical surface on a base comprising the steps of: providing a toolhaving at least a section of a substantially cylindrically perfectsurface disposed outwardly thereof; providing a flexible piece of sheetmaterial and drawing it into intimate contact with the exposed highlycylindrical surface of the tool; positioning the tool with the sheetmaterial over a drum support base having means presenting elevatedsupport surfaces which together present a section of a generallycylindrical surface; interposing an adhesive means in an uncured statebetween the exposed surface of the sheet and the upstanding supportingsurfaces of the support drum base and causing the sheet to become bondedto the base while simultaneously being mounted on the tool; and allowingthe adhesive to cure and harden so that the highly cylindricalconfiguration of the tool is maintained in the sheet material andthereafter releasing the sheet material from the tool such that thesheet material maintains the configuration of the tool.

Also, the invention resides in a method of compensating for supportsurface irregularities in the cylindrical section of a drum in aninternal photoplotter of the type having a beam projecting device havinga line of travel positioned at a given distance from the surface. Themethod comprises: mounting the beam projecting device on a frame havingat least three depending legs; providing a base having an upwardlyextending support surface presenting a section of a substantiallycylindrical surface and having means for receiving each of the legs ofthe projecting device support frame; suspending the frame above thesurface such that the depending ends of the frame are received but arenot supported by the means provided within the base for receiving thedepending legs of the frame; accounting for any irregularity in thesurface by causing the frame to be displaced to compensate for theirregularity; and fixing the frame against movement to the base byintroducing means between the depending portions of each of said legsand the receiving means of said base by filling the space therebetweenand allowing said filler to rigidify before releasing the frame fromsuspension thereby achieving a desired spatial relationship between thesurface and the beam projecting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the internal drum photoplotter employingthe present invention.

FIG. 2 is a perspective view of the drum support of the photoplottershown in FIG. 1.

FIG. 3 is a top plan view of the drum support of FIG. 2.

FIG. 4 is a perspective view of the drum support shown with the supportsheet removed to reveal the underlying vacuum system.

FIG. 5 shows schematically the final step in the process by which thesupport sheet is bonded to the underlying drum base using a cylindricaltool.

FIG. 6 is an end view of the tool used for applying the support sheet tothe drum base of FIG. 5.

FIG. 7 is a side elevational view of the tool of FIG. 6 looking at itfrom the right side.

FIG. 8 is a perspective view of the plotter and a jig assembly used tofix the plotter frame to the drum support.

FIG. 9 illustrates an alternative embodiment to alignment correction inmounting of the beam directing assembly to the drum base.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, a photoplotter is shown as 2 and embodies thedrum support 4 of the present invention having an outwardly facingsupport surface S. The photoplotter further includes a scanning means 8mounted to a frame 6 in accordance with one aspect of the invention soas to be fixed to the drum support and to support the scanning meansabove the drum for projecting an image in raster format on a substrate Fsupported by the drum surface S. The scanning means includes a radiantenergy or beam emitter means (not shown) fixed to the frame 6, a beamdirecting means, such as a spinner mirror 10, which both travels alongand rotates about a single axis herein indicated as Z while beingcarried for such movement by a carriage 14 disposed for movement along atrack 1 in the indicated Z axis. The mirror 10 is driven in rotationabout this axis by a drive motor 13 carried by a carriage 14, and thecarriage 14 is driven linearly along the track 1 by a carriage drivemotor 11. The beam emitting means is secured to one end of the carriageand emits a modulated light beam projected onto the spinner mirror beamwhich is directed downward toward the surface S of the drum to expose aline or point on a substrate, which in this example is the film F. Thecombined movements of the carriage 14, the controlled rotation of thespinner mirror by the drive motor 13 and the modulation of the beamprojected by the emitter means are coordinated by a controller such thatas the mirror 10 is moved along and rotated about the axis Z, an imageis exposed by the projection of individual rasters on the film F.

In accordance with the invention, it should be seen that the drumsupport 4 has a body portion 5 in which is disposed a half cylinderrecess 7 oriented about a longitudinal axis A. The term "half cylinder"means that the recess 7 has the shape of a cylinder cut along itslongitudinal extent through its central axis, in this case being axis A.In addition to its body portion, the drum support 4 has two outwardlyextending flange portions 20,20 each generally disposed along theopposed lateral side edges of the recess 7 and integrally connectedtherewith. From the top as seen in FIG. 2, and as will further bediscussed in greater detail with reference to FIGS. 7 and 8, the flangeportions are box-like in configuration having a floor B and arethemselves divided into four compartments 21,21 which receive thedepending legs of the scanner support frame 6 in a manner that willhereinafter be discussed. These flange portions further allow the drumto be supported by stands 22,22 which are inturn supported on a surface24, such as provided by a table or a floor.

As shown in FIG. 4, the drum support 4 is fabricated from a two-partconstruction, with the first part or base 31 being the previouslydiscussed combination of the base and flange portions and the secondpart 33 being constituted by a skin or thin sheet of material attachedto the first part through the intermediary of an attachment means 32interposed therebetween. The support sheet 33 has one face U facing theunderlying base 31 and an opposite other face S disposed outwardly uponwhich is supported the substrate F. For holding the substrate F onto andagainst the surface S of the skin, a plurality of pin holes 36,36 areprovided in it and are disposed in rows and in a given orientation so asto communicate with a vacuum means 35 provided below it for holding thesubstrate down onto the surface during a plotting operation.

The vacuum means 35 making up part of the base 31 is defined in part bya plurality of arcuately shaped ribs 37,37 spaced from one another bythe distance D, and each presenting an associated elevated surface 39,39which together define the generally half cylindrical shape of the recess7 and provide a bed upon which the sheet 33 rests. In other words, thesearcuate surfaces have a centers of curvature which are substantiallycoincident with one another so as to define the central axis A of therecess 7. Below the elevated surfaces 39,39 of the ribs is disposedchannel means 43,43 for communicating vacuum along circumferential pathsunderlying the correspondingly situated rows of the pin holes 36,36 inthe sheet 33. The ribs are interrupted approximately midlength by gaps40,40 allowing air to migrate longitudinally thereof from two ducts41,41 formed in the base 31 through which a vacuum source (not shown) isintroduced. For blocking the end interface between the sheet 33 and theopen sides of the channel means 43,43, an angle member 51 is bonded atthis juncture. The drum with its ribs and flanges may be formed in avariety of different ways and from different materials, but in thepreferred embodiment, it is cast as a unitary piece from aluminum. Insum it should be seen that the sheet 33 is supported by the ribs 37,37so as to maintain its desired cylindricity of shape. This is effectedthrough the intermediary of the attachment means 32 disposed between thelower sheet surface U and the elevated surfaces 39,39 which holds thesheet 33 in place by virtue of an adhesive bond therebetween.

Referring now to FIGS. 5-7, and particular to the method by which thesupport sheet 33 is attached to the base part, it should be seen that atool 50 is provided for mounting the sheet 33 to it for subsequentapplication onto the base 31. The tool 50 includes a clamp means 52disposed on one side thereof for securing one lateral side edge 51 ofthe sheet 33 against movement to it. The tool 50 further includes atensioning means 54 disposed diametrically oppositely of the clamp means52 and adapted to releasably hold the other opposite side edge 53 of thesheet 33. For this purpose, the sheet 33 is provided along its otherside edge 53 with a plurality of openings which are so sized andpositioned as to receive pins 60,60 carried by the tensioning means 54.Upon appropriately manipulating the means 54, the sheet 33 is caused tobe drawn into intimate contact with the outer cylindrical surface of thetool. The tensioning means 54 can take many forms, but in the preferredembodiment it comprises a bar 62 which is threadily engaged by screwsrotatably mounted in a flange 64 fixed diametrically oppositely of theclamp means 52.

It should be appreciated here that the outer surface of the tool 50 isprecision machined to have a highly accurate right angular cylindricitysuch that by pulling the sheet 33 tightly across its outer surface, thesheet is made to conform to this shape so long as it continues to betensioned. To this end, the material constituting the sheet 33, whichmay for example, be formed from aluminum and given a relatively thinthickness of about 0.02 inch, is therefore flexible enough to bemanipulated in this manner, yet is resistant to elongation relative tothe tension applied by the means 54. Prior to being mounted on the tool50 in this manner, the sheet material 33 is first conditioned by causingthe openings 36,36 to be formed by drilling them in rows which willultimately become aligned with the location of the channel means 43,43in the base 31.

The means 32 for attaching the sheet 33 to the base 31 is next appliedto the elevated surfaces 39,39 of the ribs 37,37 in order to effectbonding therebetween. The means 32 may take many different forms, but inthe preferred embodiment of the invention, it is applied as a layer ofcurable adhesive. One example of such an adhesive suitable for thispurpose is one made by EMERSON AND CUMING, INC., under the product name"Eccobond 45SC Black" (resin) and 15SC Black (catalyst). Once the sheet33 is mounted to the tool 50 in the manner discussed above, and beforethe means 32 cures, the tool 50 is then suspended over the base part 31of the drum by an appropriate suspension means and is thereafter lowereddownward toward the ribs just slightly enough to bring the lower surfaceU of the sheet 33 into engagement with the layer of adhesive disposed onthe elevated surfaces 39,39. However, once the means 32 cures andhardens, the sheet 33 is released from the tool 50 by loosening of theclamp and tensioning means 52,54. With the attachment means 32 nowholding the sheet 33 in the same configuration in which it was heldpreviously by the tool 50, a support surface is formed possessing thedesired cylindricity.

While the foregoing method of fabricating the drum 4 has been found tobe highly accurate with certain tolerances, it its a further aspect ofthe invention to provide a method by which irregularities outside of agiven tolerance can be accounted for by fixturing the scanning frame 6before fixing it to the support. That is, ideally speaking, the Z axisof the scanning means 8 should be one in the same with the A axis of thedrum 4, but due to surface irregularities which may exist, these twoaxes are not perfectly continuous with one another. Thus by adjustingthe orientation of the scanning means 8 relative to the surface S, acoarse mechanical compensation may first be made for any excessivesurface irregularity. For this purpose, as shown in FIGS. 8 and 9, analignment compensation system is provided wherein the frame 6 with thescanning means attached to it is suspended above the drum 4 at its firstend 75 by first jig 70 and at its second end 77 by a second jig 72 suchthat the legs 79,79 of the frame 6 do not rest on the bottom surface ofthe compartments 21,21 associated with the opposed flange portions20,20. The first jig 70 supports the frame at a single contact point V1while the second jig 72 supports the frame at two contact points V2 andV3, thus allowing individual vertical linear and planar adjustments tobe achieved. In addition to the vertical adjustment means provided byeach of the first and second jigs, a lateral or side to side adjustmentmeans 87 is also provided. This means may take various different forms,but in the preferred embodiment, they employ cross roller slides whichare lead screw driven. In this way, the combined affects of the firstand second and second jigs 70 and 72 allow for adjustment to be effectedin four degrees of freedom, namely in a vertical linear VL, horizontallinear HL, pitch P, and skew S. For sensing the actual cylindricity ofthe surface S, a gage G is secured to the axis of the spinner mirror andthe probe is swung through an arc M extending perpendicularly to the Zaxis at different locations along the travel path of the carriage 14.Any detected variation from a given tolerance is compensated for byadjusting one or all three vertical adjustments and/or one or both ofthe lateral adjustments provided for in each of the first and secondjigs.

As an alternative to the above process, as shown in FIG. 8, the scanningmeans track 1 may be suspended above the drum 4 such that it is ineffect weightless. This may be done by using a conventional system oflevers and counterweights. A tool 80 is provided for readilyestablishing the average location of the central axis of the surface S.For this, the tool 80 comprises two U-shaped end plates 90,90 connectedto one another by longitudinally extending connecting members 86,86which hold the end plates in a parallel relationship. The bottom of theplates are provided with a seating means 91,91, such as a soft plasticmaterial, which causes the end plates to assume the contour of thesurface which supports them. Between the two end plates is provided anindicator means in the form of a rod 86 or opposed centering bars 93,93which run center span of the two end plates. Accordingly, when the tool80 is placed on the surface S of the drum, the indicating means presentsan average central axis based upon where the tool has been placed. Acorresponding mating V-shaped follower member 88 is substituted for thespinner mirror and mounted to the carriage 14 and is engaged along therod 89 or caused to span the bars 93,93 thereby translating theorientation of the track 1 to that dictated to it by the action of thefollower member on the indicating means.

Once alignment of the tracking system is made in either of the twomanners discussed above, a filler in liquid form is poured into each ofthe compartments 21,21 that receive the footings of the legs that dependfrom the support frame 6. The filler is poured in liquid form, buthardens to a solid condition fixing the legs 79,79 relative to the base31 in the desired fixtured orientation. One example of a suitable fillerfor this purpose is made by ITW PHILADELPHIA RESINS and sold under thename SUPER ALLOY SILVER 500.

Accordingly the invention has been described by way of illustrationrather than limitation.

We claim:
 1. A method of forming a highly cylindrical surface on a base,said method comprising:providing a tool having at least a section of asubstantially cylindrically perfect surface disposed outwardly thereof;providing a flexible piece of sheet material and drawing it intointimate contact with the highly cylindrical surface of said tool;positioning said tool with said sheet material mounted to it anddirected over a drum support base having a partially cylindrical recessdefined by elevated support surfaces which together present a section ofa generally cylindrical surface; interposing an adhesive means in anuncured state between said sheet material and the elevated supportsurfaces of said support drum base and causing the sheet to becomebonded to said base while simultaneously being mounted on said tool; andallowing said adhesive to cure and harden while the sheet materialremains mounted to the tool so that the highly cylindrical configurationof said tool is maintained in the sheet material and thereafterreleasing said sheet material from said tool such that the sheetmaterial maintains the configuration of the tool.
 2. A method as definedin claim 1 further characterized by the step of providing a plurality ofsemicircular rib members as a means for providing said elevated supportsurfaces, each rib being spaced apart from one another by a givendistance; andproviding a plurality of pin holes in said sheet materialcorresponding to the location of spacings between said rib members.
 3. Amethod as defined in claim 2 further characterized in that said adhesivemeans is an epoxy which is air curable.
 4. A method as defined in claim1 further characterized by the step of drawing the sheet material intointimate contact with the outer exposed surface of said tool includingthe step of providing a tensioning means which pulls said sheet materialfrom one end while its other opposite end is clamped to said tool.